Which works well with Lucid. However, when I test the precise installer, I get a message
saying dm-multipath.ko couldn't be loaded and it might be a parameter problem. This
module doesn't take any parameters. After hitting continue life goes on until I get
to the partman section which bails because there's no mpath0.

I tested without setting auto/disk just to see and noted it was only presenting sd devices, it doesn't
look like multipath was ever loaded.

I wonder if the requisite modules are even on the installer anymore? The install
is being performed over PXE.

The dm-multipath was built-in in Lucid and became a module at some point. This needs to be added to the d-i section for the kernel build to make it available for the installer.
The device handlers never seem to have been built-in or contained in any d-i file. On the other hand it should not hurt to have them packaged as well (except space considerations on the media, but all four take about 100KB for amd64). And it is likely a bit late to experiment now.

So proposing to add dm-multipath to the md-modules udeb and the device handlers to the scsi-modules udeb.

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